Visually Complete aims to indicate what a customer will perceive as the point in time when a site looked complete.
It’s calculated by recording a video of a page loading, which is then broken down into a series of screenshot frames. A diffing image algorithm compares and marks visual completion at the frame with the least amount of difference to the final frame.
Visually complete is not a faultless metric—sites featuring animating carousels or auto-playing video content can make automated calculation extremely difficult. In these circumstances, it may be better to rely on First Contentful Paint or Largest Contentful Paint. These metrics differ in that they are focused around the initial render of the page.
If you’re looking for a metric that describes when a page is ready for interaction, Time to Interactive may be a better fit.